1. Field of the Invention
The present invention relates to a magnetic head having a spin torque oscillator, and a magnetic recording device.
2. Related Art
In the 1990's, the recording density and capacity of HDDs (Hard Disk Drives) dramatically increased, with MR (Magneto-Resistive effect) heads and GMR (Giant Magneto-Resistive effect) heads being put into practical use. However, the problems of heat fluctuations of magnetic recording media became apparent in the early 2000's, and the increase of the recording density temporarily slowed down. In 2005, perpendicular magnetic recording, which is more suitable for high-density recording than in-plane magnetic recording in principle, was put into practical use. Since then, the recording density of HDDs has been increasing at an annual rate of approximately 40%.
The latest examinations on recording density show that the recording density of 400 Gbits/inch2 has been reached. If the growth continues at this rate, the recording density of 1 Tbits/inch2 will be achieved around the year 2012. However, achieving such a high recording density is not easy by the perpendicular magnetic recording method by itself, as the problem of heat fluctuations has resurfaced.
To counter this problem, a “high-frequency assisted magnetic recording method” has been suggested. By the high-frequency assisted magnetic recording method, a high-frequency magnetic field at a frequency in the neighborhood of the resonant frequency of a magnetic recording medium, which is much higher than the recording signal frequency, is locally applied to the magnetic recording medium. As a result, the magnetic recording medium where the high-frequency magnetic field applied resonates, and its coercivity (Hc) decreases to half the original value. Having a high-frequency magnetic field been overlapped with the recording magnetic field, magnetic recording on a magnetic recording medium having higher coercivity (Hc) and greater magnetic anisotropic energy (Ku) can be feasible (see U.S. Pat. No. 6,011,664, for example). However, according to U.S. Pat. No. 6,011,664, a high-frequency magnetic field is generated with a coil. By this method, the intensity of the high-frequency magnetic field that can be applied to the recording area rapidly decreases, as the recording area on the magnetic recording medium is made smaller so as to increase the recording density. Therefore, it is difficult to reduce the coercivity of the recording area.
To counter this problem, a method of utilizing spin torque oscillators has been suggested (see United States Patent Application Publication Nos. 2005/0023938 and 2005/0219771, for example). According to United States Patent Application Publication Nos. 2005/0023938 and 2005/0219771, spin torque oscillators are formed with a stacked film consisting of a spin polarization layer, a nonmagnetic layer provided on the spin polarization layer, and a spin oscillation layer provided on the nonmagnetic layer. When a direct current is applied to the spin torque oscillators, the electron spins passing through the spin polarization layer are polarized. The polarized spin current applies a spin torque to the spin oscillation layer, so that the magnetization of the spin oscillation layer has ferromagnetic resonance. As a result, a high-frequency magnetic field is generated from the spin oscillation layer.
This phenomenon can be often observed if the device size is several tens of nanometers or less. Therefore, the region where the high-frequency magnetic field generated from the spin torque oscillators is applied is limited to a very small area at a distance of several tens of nanometers from the spin torque oscillators. A magnetic recording head with the spin torque oscillators, whose oscillation frequency is set at a value equal to or in the neighborhood of the ferromagnetic resonant frequency of the recording layer of the magnetic recording medium, should be provided near the recording magnetic pole and the magnetic recording medium. With this arrangement, the high-frequency magnetic field generated from the spin torque oscillators can be applied only to the very small recording area on the recording layer of the magnetic recording medium. As a result, only the coercivity of the very small recording area can be reduced.
When the coercivity is reduced, the magnetization of the recording area can be reversed or information writing can be performed by applying a recording magnetic field to the recording area with the use of the recording magnetic pole.
Meanwhile, there is a method of performing recording on a magnetic recording medium having high coercivity (Hc) with the use of a diagonal recording magnetic field. According to the Stoner-Wohlfarth model, in a case of a magnetic field in a 45-degree direction, magnetic reversal of a magnetic recording medium having high coercivity (Hc) can be achieved with a small recording magnetic field. By the perpendicular magnetic recording method, it is possible to generate a diagonal recording magnetic field from a plane perpendicular to the plane of the recording magnetic pole facing the recording medium. To generate a diagonal magnetic field having a rapid field intensity change, an auxiliary magnetic pole can be effectively provided near the recording magnetic pole. The gap distance between the plane perpendicular to the plane of the recording magnetic pole facing the recording medium and the plane perpendicular to the plane of the auxiliary magnetic pole facing the recording medium is adjusted so that a magnetic field is diagonally generated in the recording medium and a rapid intensity change can be achieved. Accordingly, a high-density recording can be performed with a magnetic recording head having a recording magnetic pole and an auxiliary magnetic pole. In this manner, a magnetic recording medium having higher coercivity (Hc) and greater magnetic anisotropic energy (Ku) can be used.
To effectively reduce the coercivity of the recording area by applying a high-frequency magnetic field generated from the spin torque oscillators, the oscillation frequency of the spin torque oscillators needs to be substantially equal to the ferromagnetic resonant frequency of the magnetic recording medium. The oscillation frequency of the spin torque oscillators increases in proportion to the effective magnetic field Heff applied to the spin oscillation layer. This effective magnetic field Heff is determined by an internal magnetic field (such as the magnetic anisotropy) of the spin oscillation layer and an external magnetic field (such as a recording magnetic field). In a case where the spin torque oscillators are placed near the recording magnetic pole, the magnetic field applied from the recording magnetic pole to the spin torque oscillator has a value as large as several kOe. As a result, the recording magnetic field applied to the spin torque oscillators becomes much greater than the internal magnetic field, and the oscillation frequency of the spin torque oscillators varies with the direction of the write magnetic field.
The present invention has been made in view of these circumstances, and an object thereof is to provide a magnetic head that can restrict the variation of the oscillation frequency of a spin torque oscillator placed in the vicinity of the recording magnetic pole, and a magnetic recording device that has the magnetic head.
A magnetic head according to a first aspect of the present invention includes: a recording magnetic pole to generate a recording magnetic field; a spin torque oscillator formed in the vicinity of the recording magnetic pole; and a magnetic field applying unit configured to apply a magnetic field to the spin torque oscillator, the magnetic field applied to the spin torque oscillator by the magnetic field applying unit being perpendicular to a recording magnetic field generated from the recording magnetic pole.
A magnetic head according to a second aspect of the present invention includes: a recording magnetic pole to generate a recording magnetic field; a spin torque oscillator formed in the vicinity of the recording magnetic pole; and a magnetic field applying unit provided at either end portion of the spin torque oscillator in a direction perpendicular to a direction parallel to a line connecting the recording magnetic pole and the spin torque oscillator in a plane parallel to an air bearing surface, the magnetic field applying unit configured to apply a magnetic field to the spin torque oscillator, the spin torque oscillator including: a first magnetic layer comprising at least one layer of magnetic film; a second magnetic layer comprising at least one layer of magnetic film; an intermediate layer provided between the first magnetic layer and the second magnetic layer; and an electrode comprising a first electrode layer placed on a face of the first magnetic layer on the opposite side from the intermediate layer, and a second electrode layer placed on a face of the second magnetic layer on the opposite side from the intermediate layer, the electrode being capable of applying a current flowing in a direction perpendicular to film planes of the first magnetic layer, the intermediate layer, and the second magnetic layer, the first electrode layer, the first magnetic layer, the intermediate layer, the second magnetic layer, and the second electrode layer being stacked in a direction parallel to a direction connecting the two end portions of the spin torque oscillator.
A magnetic head according to a third aspect of the present invention includes: a recording magnetic pole to generate a recording magnetic field; a spin torque oscillator formed in the vicinity of the recording magnetic pole; and a magnetic field applying unit provided at either end portion of the spin torque oscillator in a direction perpendicular to a direction parallel to a line connecting the recording magnetic pole and the spin torque oscillator in a plane parallel to an air bearing surface, the magnetic field applying unit configured to apply a magnetic field to the spin torque oscillator, the spin torque oscillator including: a first magnetic layer comprising at least one layer of magnetic film; a second magnetic layer comprising at least one layer of magnetic film; an intermediate layer provided between the first magnetic layer and the second magnetic layer; and an electrode comprising a first electrode layer placed on a face of the first magnetic layer on the opposite side from the intermediate layer, and a second electrode layer placed on a face of the second magnetic layer on the opposite side from the intermediate layer, the electrode being capable of applying a current flowing in a direction perpendicular to film planes of the first magnetic layer, the intermediate layer, and the second magnetic layer, the first electrode layer, the first magnetic layer, the intermediate layer, the second magnetic layer, and the second electrode layer being stacked in a direction perpendicular to a direction connecting the two end portions of the spin torque oscillator.
A magnetic recording device according to a fourth aspect of the present invention includes: a magnetic recording medium; and the magnetic head according to any one of first to third aspects, wherein writing on the magnetic recording medium is performed with the use of the magnetic recording head.